Alkylated phenols are widely employed as antioxidants in a broad spectrum of products, spanning the gamut from foods to fuel oils. Among such alkylated phenols it has been found that 2,4,6-trialkylphenols are especially effective antioxidants in fuel oils, and in particular 2,4,6-triisopropylphenol is an excellent stabilizer for a broad range of fuel oils, including gasoline.
The 2,4,6-trialkylphenols can be prepared by several methods which are variations on the theme of alkylating phenol with an olefin over a bed of solid catalyst, such as alumina or silica, at an appropriate temperature. However, such methods have the inherent limitation that yields of 2,4,6-trialkylphenol in excess of about 70% are difficult, if not impossible, to achieve. Yet it is an economic imperative, not merely an economic desirability, that yields be in excess of 70% because other products of the alkylation reaction are substantially less effective as antioxidants than the 2,4,6-trialkylphenol.
It might appear that one way to achieve the necessary high yield of the trialkylphenol is by separating the 2,6-dialkylphenol, which is the other major product of reaction, and further alkylating it in a separate reaction. However, it has been found that 2,6-dialkylphenols are extraordinarily difficult to alkylate. For example, 2,6-diisopropylphenol fails to undergo substantial alkylation under conditions where the 2,4,6-triisopropylphenol is formed from phenol and propylene. Stated concisely, a problem in achieving the necessary high yields of 2,4,6-triisopropylphenol by alkylating the 2,6-diisopropylphenol by-product is that the 2,6-product is relatively inert under reaction conditions.
This invention rests on a solution to the prior described problem. In particular, the solution is based on the discovery that 2,6-diisopropylphenol undergoes transalkylation with phenol under reaction conditions, leading to the desired 2,4,6-triisopropylphenol. What is most surprising is that alkylation of a mixture of 2,6-diisopropylphenol and phenol literally affords synergistic results. For example, under a particular set of reaction conditions phenol can be converted to a mixture of roughly equal amounts of 2,6-diisopropylphenol and 2,4,6-triisopropylphenol. Under the same set of conditions, 2,6-diisopropylphenol is inert. Yet, if a 50-50 mixture of phenol and 2,6-diisopropylphenol is alkylated under these conditions, one obtains 30% of 2,6-diisopropylphenol and 70% 2,4,6-triisopropylphenol, whereas one would expect to obtain 75% 2,6-diisopropylphenol and 25% 2,4,6-triisopropylphenol were the alkylation reactions independent. This is shown schematically below. EQU Experimental: 2 parts phenol.fwdarw.1 part 2,6-+1 part 2,4,6- EQU Experimental: 2 parts 2,6-.fwdarw.2 parts 2,6- EQU Calculated: 2 parts phenol+2 parts 2,6-.fwdarw.3 parts 2,6-(75%)+1 part 2,4,6-(25%) EQU Experimental: 2 parts phenol+2 parts 2,6-.fwdarw.3 parts 2,6-(30%)+7 parts 2,4,6-(70%)
The invention described herein is basically a method of preparing 2,4,6-trialkylphenol from a feedstock of phenol and 2,6-dialkylphenol, the latter being recycled from the product stream. It affords the completely unexpected result in that the 2,4,6-trialkylphenol is formed from such a feestock in substantially higher yields than from either of the components alone under the same reaction conditions. In addition to the method having the important advantage of leading to higher overall yield of the desired trialkylphenol, it has the further advantage that use of 2,6-dialkylphenol as a diluent for phenol in the feedstock moderates the exotherm of the alkylation reaction. Although this latter benefit is incidental to the invention, nonetheless it is important from an engineering aspect because a sometimes serious problem attending extensive alkylation of phenol is the development of a hot spot in the continuous reactor, thereby causing oligomerization of the olefin or, in extreme cases, a runaway reaction.